On my continuing odyssey to analyze real-life circuits as a way to expand my knowledge, I'm studying this Heathkit Curve Tracer:

IT-1121 Curve Tracer Schematic

There are a couple of features about the sweep section power supply I don't fully understand. Here's a snippet of the area of interest:

enter image description here

and here's my (very simplified) re-interpretation of the entire sweep section, to make it easier to digest:

enter image description here

My questions are:

1) Ignoring the high voltage full wave rectifier for now, why does the author tie the bridge rectifier return to the secondary centre tap?

2) What is the purpose of the adjustable low-pass "loop compensation" section highlighted above? How does it work?

Note: I've just realized having laid all this out, that the loop compensation low-pass IS a point of return for the bridge, since it shares a ground point with other parts of the circuit. So, are there two paths back? At any rate, they're both tied back to the C.T., which is confusing to me.

Also, I've noticed from my own analysis, that tying the return to the tap in this way drives more current through each half of the secondary winding on each half cycle. This increases the voltage out of the bridge (possibly as a consequence of a bigger current through the reduced reactance of the half-winding??)


You may be missing the point here. This section is not a "Power" supply it is a "Sweep" supply. Its purpose is to provide two wave forms (derived from the AC line wave form) to the horizontal and vertical driver circuits.

The two wave forms shown in the diagram are the important features of the circuit.

Tying the bridge rectifier group to the center tap gives the signal a 0v reference.

The loop compensation potentiometers possibly provides some small adjustment to the wave form's phase.

The low value capacitors in this section are being used to help smooth out the wave forms, not to create a DC supply.

Applying simple waveforms like this to an oscilloscope XY inputs can create a two dimensional pattern. In this case the wave form can drive the test transistor which causes the characteristic curve patterns to be displayed on the oscilloscope.

In the original schematic the power supply to the system is shown on the transformer below this section, (see: Low Voltage Power Supply).

  • \$\begingroup\$ I'm quite aware that the circuit produces a rectified alternating waveform. I never mentioned DC in my question and as you can see from my simplified schematic, I'm aware it produces two measurements. What I asked was why the bridge was designed in such an "unconventional" way, with it's return tied back to the centre tap. Also, I'm not so sure about the low value caps "smoothing" the waveform. What little information there is about this compensation suggests it's there to make up for stray capacitances in the transfromer, etc. \$\endgroup\$ – Buck8pe Apr 7 '16 at 12:48
  • \$\begingroup\$ I'm thinking some kind of "phase adjust", but that's just a wild guess. \$\endgroup\$ – Buck8pe Apr 7 '16 at 12:49
  • \$\begingroup\$ @Buck8pe, the caps connected directly across the rectifiers provide some smoothing action to the wave forms. Stray capacitance or other imperfections in the transformer could create small phase offsets, so the compensation RC combination can adjust for these offsets. My reference to DC was just that bridge rectifiers are more often used to create DC power supplys, (and the fact that your questions title lists "Power Supply" puzzle). \$\endgroup\$ – Nedd Apr 7 '16 at 13:25
  • \$\begingroup\$ Just missed your updated answer, I'm not sure I understand your comment about a 0V reference. 0V relative to ground? I can see 0V written on the schematic, but my simulations show negative peaks of over 40V (relative to the ground points given). I'm obviously missing something. \$\endgroup\$ – Buck8pe Apr 7 '16 at 13:30
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    \$\begingroup\$ 0v in reference to the wave form, not necessarily circuit ground. The wave form will be an AC signal relative to that point. So with that point used as the vertical input the horizon signal will be relative to that point. (Edit to prior comment.). -hope this helped, have a nice night. \$\endgroup\$ – Nedd Apr 7 '16 at 13:56

"Looping" is a phenomenon where the displayed trace exhibits a "loop" when the voltage in the collector supply increases and decreases. In a perfect system, the voltage rises (like half a sine wave) going from 0 to whatever maximum value the collector supply is set to and returns on EXACTLY the same path. You see a nice thin line being drawn. Because there's always some stray capacitance in the system, the rising voltage charges that capacitance, which means that the current on the descending sine wave half isn't the same as the current as the voltage rises. You see that as a loop on the display instead of a thin line. Most systems offer some amount of leakage capacitance to swamp out the system capacitance. Sometime it's variable and sometimes the amount is fixed but typically some looping compensation is included to minimize looping effects.


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